Primarily because of their use in optical communications systems, the fabrication of optical fibers has been a subject of intensive research and development. Optical fibers are typically made in a continous process which involves drawing a thin glass strand or fiber from a heat-softened glass preform, and thereafter coating it with a polymer to increase its structural strength. The Andrejco U.S. Pat. No. 4,450,333, assigned to AT&T Technologies, Inc., hereby incorporated herein by reference, describes in detail a furnace of the type that can be used to melt a glass preform to permit a fiber to be drawn.
The paper of Smithgall and Frazee, "High Speed Measurement and Control of Fiber-Coating Concentricity," the Bell System Technical Journal, Vol. 60, No. 9, November 1981, pp. 2065-2080, hereby incorporated by reference herein, describes a method for monitoring the concentricity of the optical fiber polymer coating by directing orthogonal light beams at the coated fiber. Each of the beams impinging on the fiber results in a pattern of forward-scattered light modes that are projected on orthogonally constructed screens. Two television cameras are used to detect these patterns projected on the screens, and more specifically, to detect asymmetries in the light distribution which indicates a deviation from concentricity of the polymer coating. Thus, each of the television cameras generates signals indicative of any deviation from concentricity which are in turn used to make concentricity adjustments.
The copending patent application of Frazee et al., Ser. No. 316,192, filed Feb. 27, 1989, hereby incorporated by reference herein, describes a method for making, between the glass portion of the optical fiber and the polymer coating, a thin carbon coating which can hermetically seal the glass optical fiber to increase its structural strength and its resistance to environmental contaminants. The Frazee et al. patent application further describes a method for monitoring the thickness of the carbon coating by using the same orthogonal optical beam arrangement as described in the Smithgall et al. paper. However, as described in the Frazee et al. application, the carbon coating thickness is related to forward-scattered mode intensity. As a consequence, the strength of the signals generated by the two television cameras may be used to provide an indication of absolute light mode intensity. If the intensity falls outside of prescribed ranges, the signals are used to adjust automatically the thickness of the applied carbon coating so as to maintain it within a prescribed range of thicknesses. Thus, the signals from the two television cameras are used to make both polymer coating concentricity adjustments and carbon coating thickness adjustments in real time during the optical fiber fabrication.
While the cost of finished usable optical fiber has fallen dramatically in the last five years, continuing efforts are being made to reduce the cost further. Worldwide competition in this technology is extremely vigorous, and it is clear that any improvement that will reduce the cost of making the finished product without sacrificing quality will be diligently pursued.